1. Field of the Invention
The present invention relates to an angular velocity sensor correcting technique and, more particularly, to an angular velocity sensor correcting apparatus and an angular velocity sensor correcting method for deriving a value for correcting an output signal from an angular velocity sensor.
2. Description of the Related Art
In a navigation apparatus for a vehicle, an optimum position is generally estimated by combining a position calculated by self-contained navigation and a position calculated from a Global Positioning System (GPS). In the self-contained navigation, by updating a measured position of last time on the basis of a speed pulse indicative of the velocity of the vehicle and turn angle velocity of the vehicle measured by an angular velocity sensor, the present position is calculated. By a navigation apparatus of such a system, the position of the vehicle can be derived by self-contained navigation even in a tunnel, an underground parking area, and an area between tall buildings where it is difficult to receive radio waves from a GPS satellite. An angular velocity ω accompanying a turn of a vehicle is derived by the following equation:ω=(Vout−Voffset)/S  (1)In the equation, Vout denotes output voltage of an angular velocity sensor, Voffset denotes an offset value of the angular velocity sensor, and S (mV/deg/sec) denotes an angular velocity conversion coefficient of the angular velocity sensor.
To accurately obtain angular velocity, an offset value and the angular velocity conversion coefficient of the angular velocity sensor have to be obtained accurately. The angular velocity conversion coefficient of the angular velocity sensor varies, generally, according to the individual difference of the angular velocity sensor and the angle of attachment to the vehicle of the angular velocity sensor. There is the possibility that the offset value changes according to a temperature change. That is, the offset value is subjected to the influences of temperature rise caused by heat generation of a substrate or the like used for a navigation apparatus for a vehicle and heat generation of a vehicle engine or the like in the case where the navigation apparatus for a vehicle is attached to the dashboard of the vehicle. Hitherto, an offset value of an angular velocity sensor is corrected by using an output voltage from the angular velocity sensor at the time of stop or straight running of the vehicle when the angular velocity is “0”. However, in the case where the frequency of stop of the vehicle is low such as running on an expressway or long-time running in an area where a traffic amount is small, it is difficult to periodically correct the offset value of the angular velocity sensor. The precision of the offset value tends to deteriorate. Correction of an offset value at the time of straight running when an output voltage from an angular velocity sensor accurately is “0” is easily influenced by the shape of a road and the driving state of the driver, so that it is difficult to periodically perform the correction. The angular velocity conversion coefficient of the angular velocity sensor is derived from an orientation change amount in a unit period and an output voltage of the angular velocity sensor. Consequently, as it is obvious from the equation (1), the angular velocity conversion coefficient of the angular velocity sensor is influenced by an error of the offset value.
A technique of correcting an offset of the angular velocity sensor and the angular velocity conversion coefficient even during running other than the straight running is proposed. In the technique, on the basis of an average value of output voltages of the angular velocity sensor in a predetermined period and the orientation change amount of the vehicle in the period in which the average value is calculated, the offset value and the angular velocity conversion coefficient of the angular velocity sensor are corrected. Concretely, an offset value Voffset of the angular velocity sensor is derived as follows:Voffset=1/n·ΣVout−1/Δt·Δθ/n·S  (2)In the equation, “n” denotes the number of samples of the output voltage of the angular velocity sensor, Δt (sec) denotes sampling interval, and Δθ(deg) denotes an orientation change amount. The orientation change amount is obtained on the basis of a GPS orientation obtained from a GPS satellite or map data. The angular velocity conversion coefficient of the angular velocity sensor is derived as follows in a state where a change amount of a corrected offset amount is small, that is, in a stable state:S=(1/n·ΣVout−Voffset)·n/Δθ·Δt  (3)In the equation, Voffset is known and a constant in a stable state where a change amount of a corrected offset amount is small as illustrated in, for example, laid open Japanese patent application number 2001-330454.
Under such situations, when the number of samples is increased since Voffset in the equation (3) is not stable until the stable state is obtained in which the change amount of the offset value is small with respect to the angular velocity conversion coefficient of the angular velocity sensor, there is the possibility that an error in the angular velocity conversion coefficient increases. Until the offset value becomes stable, generally, it takes 30 minutes to one hour or longer since startup. Consequently, for example, in running of about 30 minutes or less, it is very difficult to accurately correct the angular velocity conversion coefficient of the angular velocity sensor, and precision of deriving the angular velocity is not high. To improve the precision of deriving the angular velocity in short time since startup, it is requested to derive the offset value of the angular velocity sensor in short time.